path: root/lib/kernel/doc/src/logger_chapter.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2017</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>Logging</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>logger_chapter.xml</file>
  </header>

  <section>
    <title>Overview</title>
    <p>Erlang/OTP provides a standard API for logging. The backend of
      this API can be used as is, or it can be customized to suite
      specific needs.</p>
    <p>It consists of two parts - the <em>logger</em> part and the
      <em>handler</em> part. The logger will forward log events to one
      or more handler(s).</p>

    <image file="logger_arch.png">
      <icaption>Conceptual overview</icaption>
    </image>

    <p><em>Filters</em> can be added to the logger and to each
      handler. The filters decide if an event is to be forwarded or
      not, and they can also modify all parts of the log event.</p>

    <p>A <em>formatter</em> can be set for each handler. The formatter
      does the final formatting of the log event, including the log
      message itself, and possibly a timestamp, header and other
      metadata.</p>

    <p>In accordance with the Syslog protocol, RFC-5424, eight
      severity levels can be specified:</p>

    <table align="left">
      <row>
	<cell><strong>Level</strong></cell>
	<cell align="center"><strong>Integer</strong></cell>
	<cell><strong>Description</strong></cell>
      </row>
      <row>
	<cell>emergency</cell>
	<cell align="center">0</cell>
	<cell>system is unusable</cell>
      </row>
      <row>
	<cell>alert</cell>
	<cell align="center">1</cell>
	<cell>action must be taken immediately</cell>
      </row>
      <row>
	<cell>critical</cell>
	<cell align="center">2</cell>
	<cell>critical contidions</cell>
      </row>
      <row>
	<cell>error</cell>
	<cell align="center">3</cell>
	<cell>error conditions</cell>
      </row>
      <row>
	<cell>warning</cell>
	<cell align="center">4</cell>
	<cell>warning conditions</cell>
      </row>
      <row>
	<cell>notice</cell>
	<cell align="center">5</cell>
	<cell>normal but significant conditions</cell>
      </row>
      <row>
	<cell>info</cell>
	<cell align="center">6</cell>
	<cell>informational messages</cell>
      </row>
      <row>
	<cell>debug</cell>
	<cell align="center">7</cell>
	<cell>debug-level messages</cell>
      </row>
      <tcaption>Severity levels</tcaption>
    </table>

    <p>A log event is allowed by Logger if the integer value of
      its <c>Level</c> is less than or equal to the currently
      configured log level. The log level can be configured globally,
      or to allow more verbose logging from a specific part of the
      system, per module.</p>

    <section>
      <title>Customizable parts</title>

      <taglist>
	<tag><marker id="Handler"/>Handler</tag>
	<item>
	  <p>A handler is defined as a module exporting the following
	    function:</p>

	  <code>log(Log, Config) -> ok</code>

	  <p>A handler is called by the logger backend after filtering on
	    logger level and on handler level for the handler which is
	    about to be called. The function call is done on the client
	    process, and it is up to the handler implementation if other
	    processes are to be involved or not.</p>

	  <p>Multiple instances of the same handler can be
	    added. Configuration is per instance.</p>

	</item>

	<tag><marker id="Filter"/>Filter</tag>
	<item>
	  <p>Filters can be set on the logger or on a handler. Logger
	    filters are applied first, and if passed, the handler filters
	    for each handler are applied. The handler plugin is only
	    called if all handler filters for the handler in question also
	    pass.</p>

	  <p>A filter is specified as:</p>

	  <code>{fun((Log,Extra) -> Log | stop | ignore), Extra}</code>

	  <p>The configuration parameter <c>filter_default</c>
	    specifies the behavior if all filters return <c>ignore</c>.
	    <c>filter_default</c> is by default set to <c>log</c>.</p>

	  <p>The <c>Extra</c> parameter may contain any data that the
	    filter needs.</p>
	</item>

	<tag><marker id="Formatter"/>Formatter</tag>
	<item>
	  <p>A formatter is defined as a module exporting the following
	    function:</p>

	  <code>format(Log,Extra) -> string()</code>

	  <p>The formatter plugin is called by each handler, and the
	    returned string can be printed to the handler's destination
	    (stdout, file, ...).</p>
	</item>

      </taglist>
    </section>

    <section>
      <title>Built-in handlers</title>

      <taglist>
	<tag><c>logger_std_h</c></tag>
	<item>
	  <p>This is the default handler used by OTP. Multiple instances
	    can be started, and each instance will write log events to a
	    given destination, console or file. Filters can be used for
	    selecting which event to send to which handler instance.</p>
	</item>

	<tag><c>logger_disk_log_h</c></tag>
	<item>
	  <p>This handler behaves much like logger_std_h, except it uses
	    <seealso marker="disk_log"><c>disk_log</c></seealso> as its
	    destination.</p>
	</item>

	<tag><marker id="ErrorLoggerManager"/><c>error_logger</c></tag>
	<item>
	  <p>This handler is to be used for backwards compatibility
	    only. It is not started by default, but will be automatically
	    started the first time an event handler is added
	    with <seealso marker="error_logger#add_report_handler-1">
	      <c>error_logger:add_report_handler/1,2</c></seealso>.</p>

	  <p>No built-in event handlers exist.</p>
	</item>
      </taglist>
    </section>

    <section>
      <title>Built-in filters</title>

      <taglist>
	<tag><c>logger_filters:domain/2</c></tag>
	<item>
	  <p>This filter provides a way of filtering log events based on a
	    <c>domain</c> field <c>Metadata</c>. See 
	    <seealso marker="logger_filters#domain-2">
	    <c>logger_filters:domain/2</c></seealso></p>
	</item>

	<tag><c>logger_filters:level/2</c></tag>
	<item>
	  <p>This filter provides a way of filtering log events based
	    on the log level. See <seealso marker="logger_filters#domain-2">
	    <c>logger_filters:domain/2</c></seealso></p>
	</item>

	<tag><c>logger_filters:progress/2</c></tag>
	<item>
	  <p>This filter matches all progress reports
	    from <c>supervisor</c> and <c>application_controller</c>.
	    See <seealso marker="logger_filters#progress/2">
	    <c>logger_filters:progress/2</c></seealso></p>
	</item>

	<tag><c>logger_filters:remote_gl/2</c></tag>
	<item>
	  <p>This filter matches all events originating from a process
	    that has its group leader on a remote node.
	    See <seealso marker="logger_filters#remote_gl/2">
	    <c>logger_filters:remote_gl/2</c></seealso></p>
	</item>
      </taglist>
    </section>

    <section>
      <title>Default formatter</title>

      <p>The default formatter is <c>logger_formatter</c>.
	See <seealso marker="logger_formatter#format-2">
	  <c>logger_formatter:format/2</c></seealso>.</p>
    </section>
  </section>

  <section>
    <title>Configuration</title>

    <section>
      <title>Application environment variables</title>
      <p>See <seealso marker="kernel_app#configuration">Kernel(6)</seealso> for
	information about the application environment variables that can
	be used for configuring logger.</p>
    </section>

    <section>
      <title>Logger configuration</title>

      <taglist>
	<tag><c>level</c></tag>
	<item>
	  <p>Specifies the severity level to log.</p>
	</item>
	<tag><c>filters</c></tag>
	<item>
	  <p>Logger filters are added or removed with
	    <seealso marker="logger#add_logger_filter-2">
	      <c>logger:add_logger_filter/2</c></seealso> and
	    <seealso marker="logger#remove_logger_filter-1">
	      <c>logger:remove_logger_filter/1</c></seealso>,
	    respectively.</p>
	  <p>See <seealso marker="#Filter">Filter</seealso> for more
	    information.</p>
	  <p>By default, no filters exist.</p>
	</item>
	<tag><c>filter_default = log | stop</c></tag>
	<item>
	  <p>Specifies what to do with an event if all filters
	    return <c>ignore</c>.</p>
	  <p>Default is <c>log</c>.</p>
	</item>
	<tag><c>handlers</c></tag>
	<item>
	  <p>Handlers are added or removed with
	    <seealso marker="logger#add_handler-3">
	      <c>logger:add_handler/3</c></seealso> and
	    <seealso marker="logger#remove_handler-1">
	      <c>logger:remove_handler/1</c></seealso>,
	    respectively.</p>
	  <p>See <seealso marker="#Handler">Handler</seealso> for more
	    information.</p>
	</item>
      </taglist>
    </section>

    <section>
      <title>Handler configuration</title>
      <taglist>
	<tag><c>level</c></tag>
	<item>
	  <p>Specifies the severity level to log.</p>
	</item>
	<tag><c>filters</c></tag>
	<item>
	  <p>Handler filters can be specified when adding the handler,
	    or added or removed later with
	    <seealso marker="logger#add_handler_filter-3">
	      <c>logger:add_handler_filter/3</c></seealso> and
	    <seealso marker="logger#remove_handler_filter-2">
	      <c>logger:remove_handler_filter/2</c></seealso>,
	    respectively.</p>
	  <p>See <seealso marker="#Filter">Filter</seealso> for more
	    information.</p>
	  <p>By default, no filters exist.</p>
	</item>
	<tag><c>filter_default = log | stop</c></tag>
	<item>
	  <p>Specifies what to do with an event if all filters
	    return <c>ignore</c>.</p>
	  <p>Default is <c>log</c>.</p>
	</item>
        <tag><c>depth = pos_integer() | unlimited</c></tag>
	<item>
	  <p>Specifies if the depth of terms in the log events shall
	    be limited by using control characters <c>~P</c>
	    and <c>~W</c> instead of <c>~p</c> and <c>~w</c>,
	    respectively. See
	    <seealso marker="stdlib:io#format-1"><c>io:format</c></seealso>.</p>
	</item>
        <tag><c>max_size = pos_integer() | unlimited</c></tag>
	<item>
	  <p>Specifies if the size of a log event shall be limited by
	    truncating the formatted string.</p>
	</item>
	<tag><c>formatter = {Module::module(),Extra::term()}</c></tag>
	<item>
	  <p>See <seealso marker="#Formatter">Formatter</seealso> for more
	    information.</p>
	  <p>The default module is <seealso marker="logger_formatter">
	      <c>logger_formatter</c></seealso>, and <c>Extra</c> is
	      it's configuration map.</p>
	</item>
      </taglist>

      <p>Note that <c>level</c> and <c>filters</c> are obeyed by
	Logger itself before forwarding the log events to each
	handler, while <c>depth</c>, <c>max_size</c>
	and <c>formatter</c> are left to the handler
	implementation. All Logger's built-in handlers do apply these
	configuration parameters before printing.</p>
    </section>

  </section>

  <section>
    <marker id="compatibility"/>
    <title>Backwards compatibility with error_logger</title>
    <p>Logger provides backwards compatibility with the old
      <c>error_logger</c> in the following ways:</p>

    <taglist>
      <tag>Legacy event handlers</tag>
      <item>
	<p>To use event handlers written for <c>error_logger</c>, just
	  add your event handler with</p>
	<code>
error_logger:add_report_handler/1,2.
	</code>
	<p>This will automatically start the <c>error_logger</c>
	  event manager, and add <c>error_logger</c> as a
	  handler to <c>logger</c>, with configuration</p>
<code>
#{level=>info,
  filter_default=>log,
  filters=>[]}.
</code>
	<p>Note that this handler will ignore events that do not
	  originate from the old <c>error_logger</c> API, or from
	  within OTP. This means that if your code uses the logger API
	  for logging, then your log events will be discarded by this
	  handler.</p>
	<p>Also note that <c>error_logger</c> is not overload
	  protected.</p>
      </item>
      <tag>Logger API</tag>
      <item>
	<p>The old <c>error_logger</c> API still exists, but should
	  only be used by legacy code. It will be removed in a later
	  release.</p>
      </item>
      <tag>Output format</tag>
      <item>
	<p>To get log events on the same format as produced
	  by <c>error_logger_tty_h</c> and <c>error_logger_file_h</c>,
	  use the default formatter, <c>logger_formatter</c>, with
	  configuration parameter <c>legacy_header=>true</c>. This is
	  also the default.</p>
      </item>
      <tag>Default format of log events from OTP</tag>
      <item>
	<p>By default, all log events originating from within OTP,
	  except the former so called "SASL reports", look the same as
	  before.</p>
      </item>
      <tag>SASL reports</tag>
      <item>
	<p>By SASL reports we mean supervisor reports, crash reports
	  and progress reports.</p>
	<p>In earlier releases, these reports were only logged when
	  the SASL application was running, and they were printed
	  trough specific event handlers
	  named <c>sasl_report_tty_h</c>
	  and <c>sasl_report_file_h</c>.</p>
	<p>The destination of these log events were configured by
	  environment variables for the SASL application.</p>
	<p>Due to the specific event handlers, the output format
	  slightly differed from other log events.</p>
	<p>As of OTP-21, the concept of SASL reports is removed,
	  meaning that the default behavior is as follows:</p>
	<list>
	  <item>Supervisor reports, crash reports and progress reports
	    are no longer connected to the SASL application.</item>
	  <item>Supervisor reports and crash reports are logged by
	    default.</item>
	  <item>Progress reports are not logged by default, but can be
	    enabled with the kernel environment
	    variable <c>logger_log_progress</c>.</item>
	  <item>The output format is the same for all log
	    events.</item>
	</list>
	<p>If the old behavior is preferred, the kernel environment
	  variable <c>logger_sasl_compatible</c> can be set
	  to <c>true</c>. The old SASL environment variables can then
	  be used as before, and the SASL reports will only be printed
	  if the SASL application is running - through a second log
	  handler named <c>sasl_h</c>.</p>
	<p>All SASL reports have a metadata
	  field <c>domain=>[beam,erlang,otp,sasl]</c>, which can be
	  used, for example, by filters to to stop or allow the
	  events.</p>
      </item>
    </taglist>
  </section>


  <section>
    <title>Error handling</title>
    <p>Log data is expected to be either a format string and
      arguments, a string (unicode:chardata), or a report (map or
      key-value list) which can be converted to a format string and
      arguments by the handler. A default report callback should be
      included in the log event's metadata, which can be used for
      converting the report to a format string and arguments. The
      handler might also do a custom conversion if the default format
      is not desired.</p>
    <p><c>logger</c> does, to a certain extent, check its input data
      before forwarding a log event to the handlers, but it does not
      evaluate conversion funs or check the validity of format strings
      and arguments. This means that any filter or handler must be
      careful when formatting the data of a log event, making sure
      that it does not crash due to bad input data or faulty
      callbacks.</p>
    <p>If a filter or handler still crashes, logger will remove the
      filter or handler in question from the configuration, and then
      print a short error message on the console. A debug event
      containing the crash reason and other details is also issued,
      and can be seen if a handler is installed which logs on debug
      level.</p>
  </section>

  <section>
    <title>Example: add a handler to log debug events to file</title>
    <p>When starting an erlang node, the default behavior is that all
      log events with level info and above are logged to the
      console. In order to also log debug events, you can either
      change the global log level to <c>debug</c> or add a separate
      handler to take care of this. In this example we will add a new
      handler which prints the debug events to a separate file.</p>
    <p>First, we add an instance of logger_std_h with
      type <c>{file,File}</c>, and we set the handler's level
      to <c>debug</c>:</p>
    <pre>
1> <input>Config = #{level=>debug,logger_std_h=>#{type=>{file,"./debug.log"}}}.</input>
#{logger_std_h => #{type => {file,"./debug.log"}},
  level => debug}
2> <input>logger:add_handler(debug_handler,logger_std_h,Config).</input>
ok</pre>
    <p>By default, the handler receives all events, so we need to add a filter
      to stop all non-debug events:</p>
    <pre>
3> <input>Fun = fun(#{level:=debug}=Log,_) -> Log; (_,_) -> stop end.</input>
#Fun&lt;erl_eval.12.98642416>
4> <input>logger:add_handler_filter(debug_handler,allow_debug,{Fun,[]}).</input>
ok</pre>
    <p>And finally, we need to make sure that the logger itself allows
      debug events. This can either be done by setting the global
      logger level:</p>
    <pre>
5> <input>logger:set_logger_config(level,debug).</input>
ok</pre>
    <p>Or by allowing debug events from one or a few modules only:</p>
    <pre>
6> <input>logger:set_module_level(mymodule,debug).</input>
ok</pre>

  </section>

  <section>
    <title>Example: implement a handler</title>
    <p>The only requirement that a handler MUST fulfill is to export
      the following function:</p>
    <code>log(logger:log(),logger:config()) ->ok</code>
    <p>It may also implement the following callbacks:</p>
    <code>
adding_handler(logger:handler_id(),logger:config()) -> {ok,logger:config()} | {error,term()}
removing_handler(logger:handler_id()) -> ok
changing_config(logger:handler_id(),logger:config(),logger:config()) -> {ok,logger:config()} | {error,term()}
    </code>
    <p>When logger:add_handler(Id,Module,Config) is called, logger
      will first call Module:adding_handler(Id,Config), and if it
      returns {ok,NewConfig} the NewConfig is written to the
      configuration database. After this, the handler may receive log
      events as calls to Module:log/2.</p>
    <p>A handler can be removed by calling
      logger:remove_handler(Id). logger will call
      Module:removing_handler(Id), and then remove the handler's
      configuration from the configuration database.</p>
    <p>When logger:set_handler_config is called, logger calls
      Module:changing_config(Id,OldConfig,NewConfig). If this function
      returns ok, the NewConfig is written to the configuration
      database.</p>

    <p>A simple handler which prints to the console could be
      implemented as follows:</p>
    <code>
-module(myhandler).
-export([log/2]).

log(#{msg:={report,R}},_) ->
    io:format("~p~n",[R]);
log(#{msg:={string,S}},_) ->
    io:put_chars(S);
log(#{msg:={F,A}},_) ->
    io:format(F,A).
    </code>

    <p>A simple handler which prints to file could be implemented like
      this:</p>
    <code>
-module(myhandler).
-export([adding_handler/2, removing_handler/1, log/2]).
-export([init/1, handle_call/3, handle_cast/2, terminate/2]).

adding_handler(Id,Config) ->
    {ok,Fd} = file:open(File,[append,{encoding,utf8}]),
    {ok,Config#{myhandler_fd=>Fd}}.

removing_handler(Id,#{myhandler_fd:=Fd}) ->
    _ = file:close(Fd),
    ok.

log(#{msg:={report,R}},#{myhandler_fd:=Fd}) ->
    io:format(Fd,"~p~n",[R]);
log(#{msg:={string,S}},#{myhandler_fd:=Fd}) ->
    io:put_chars(Fd,S);
log(#{msg:={F,A}},#{myhandler_fd:=Fd}) ->
    io:format(Fd,F,A).
    </code>

    <p>Note that none of the above handlers have any overload
      protection, and all log events are printed directly from the
      client process. Neither do the handlers use the formatter or
      in any way add time or other metadata to the printed events.</p>

    <p>For examples of overload protection, please refer to the
      implementation
      of <seealso marker="logger_std_h"><c>logger_std_h</c></seealso>
      and <!--<seealso marker="logger_disk_log_h"--><c>logger_disk_log_h</c>
      <!--/seealso-->.</p>

    <p>Below is a simpler example of a handler which logs through one
      single process, and uses the default formatter to gain a common
      look of the log events.</p>
    <p>It also uses the metadata field <c>report_cb</c>, if it exists,
      to print reports in the way the event issuer suggests. The
      formatter will normally do this, but if the handler either has
      an own default (as in this example) or if the
      given <c>report_cb</c> should not be used at all, then the
      handler must take care of this itself.</p>
    <code>
-module(myhandler).
-export([adding_handler/2, removing_handler/1, log/2]).
-export([init/1, handle_call/3, handle_cast/2, terminate/2]).

adding_handler(Id,Config) ->
    {ok,Pid} = gen_server:start(?MODULE,Config),
    {ok,Config#{myhandler_pid=>Pid}}.

removing_handler(Id,#{myhandler_pid:=Pid}) ->
    gen_server:stop(Pid).

log(Log,#{myhandler_pid:=Pid} = Config) ->
    gen_server:cast(Pid,{log,Log,Config}).

init(#{myhandler_file:=File}) ->
    {ok,Fd} = file:open(File,[append,{encoding,utf8}]),
    {ok,#{file=>File,fd=>Fd}}.

handle_call(_,_,State) ->
    {reply,{error,bad_request},State}.

handle_cast({log,Log,Config},#{fd:=Fd} = State) ->
    do_log(Fd,Log,Config),
    {noreply,State}.

terminate(Reason,#{fd:=Fd}) ->
    _ = file:close(Fd),
    ok.

do_log(Fd,#{msg:={report,R}} = Log, Config) ->
    Fun = maps:get(report_cb,Config,fun my_report_cb/1,
    {F,A} = Fun(R),
    do_log(Fd,Log#{msg=>{F,A},Config);
do_log(Fd,Log,#{formatter:={FModule,FConfig}}) ->
    String = FModule:format(Log,FConfig),
    io:put_chars(Fd,String).

my_report_cb(R) ->
    {"~p",[R]}.
    </code>
  </section>


  <section>
    <title>See Also</title>
    <p><seealso marker="error_logger"><c>error_logger(3)</c></seealso>,
    <seealso marker="sasl:sasl_app"><c>SASL(6)</c></seealso></p>
  </section>
</chapter>